hypothesis and standardized test statistic

had to miss my stat class the other day and i'm having trouble on my homework. i got a little but any step by step guidance would be appreciated

The Department of Economic and Community Development (DECD) reported that in 2009 the
average number of new jobs created per county was 450. The department also provided the following
information regarding a sample of 5 counties in 2010.
County New Jobs Created
In 2010
Bradley 410
Rhea 480
Marion 407
Grundy 428
Sequatchie 400
a. Compute the sample average and the standard deviation for 2010.
b. We want to determine whether there has been a significant decrease in the average number
of jobs created. Provide the null and the alternative hypotheses.
c. Compute the standardized test statistic. Assume the population is normally distributed.
d. At 5% (α = .05) significance, test the hypotheses. State your conclusion.

Your standard deviation and thus your variance is incorrect, what you're looking for is an unbiased estimate of the variance for the population. Take a look through here and I'll help you further, Unbiased and Biased Estimators